Transmission oil viscosity – SAE J306 2025 update

Viscosity is one of the most critical physical properties in the design and selection of transmission lubricants, as it directly impacts a fluid’s ability to form a protective film between mechanical surfaces, thereby reducing friction and wear. Technically, it refers to a fluid’s resistance to flow under the action of an external force.

Viscosity is also a key parameter influencing the efficiency and performance of mechanical components. In the context of improving system efficiency and reducing emissions, it has become a pivotal factor in mechanical design.

Following recent revisions to the SAE J300 categories for engine oils, a major update to the SAE J306 classification for gear and axle lubricants has now been introduced.

For lubricants intended for use in transmissions, axles and differentials, viscosity classification is defined by SAE Standard J306, issued by the Society of Automotive Engineers (SAE). This standard establishes minimum performance criteria based on kinematic viscosity at 100 °C and low-temperature viscosity.

The new 2025 revision builds upon the 2019 update, which introduced three new low-viscosity grades (SAE 65, 70, 75) aimed at supporting the development of more modern and efficient transmission systems. These changes enabled the formulation of lower-viscosity fluids without compromising mechanical protection, thereby helping to reduce energy losses and fuel consumption.

Today, the industry faces another significant shift with the introduction of a revised SAE J306 standard, marking a substantial technological evolution. This update introduces a new approach to low-temperature viscosity classification, measured using the ASTM D2983 method (Brookfield Viscosity), which defines the maximum allowable viscosity - and therefore pumpability - at a fixed temperature of -40 °C, reshaping previous “W” grade definitions.

As part of this update, four new cold-temperature viscosity grades are introduced: 62W, 63W, 64W and 65W, along with their corresponding high-temperature viscosity classifications (at 100 °C): SAE 62, 63, 64, and 65.

Previously, categories like 85W, 80W, 75W, and 70W were defined by a maximum viscosity threshold of 150,000 cP measured at progressively lower temperatures (-12 °C, -26 °C, -40 °C, and -55 °C, respectively). In contrast, the new grades set distinct maximum viscosity limits for each category—1,200 cP for 62W, 2,500 cP for 63W, 5,000 cP for 64W, and 10,000 cP for 65W, all measured at the same reference temperature of -40 °C.

These new viscosity classes fall below the minimum thresholds established in 2019, offering more refined categorization for lubricants designed for extreme operating conditions and next-generation transmission systems, including those found in electric vehicles (EVs).